Problem: Simplify the following expression: $z = \dfrac{9a}{10} + \dfrac{-a}{10}$
Explanation: Since the expressions have the same denominator we simply combine the numerators: $z = \dfrac{9a - a}{10}$ Combine like terms: $z = \dfrac{8a}{10}$ Simplify by dividing numerator and denominator by 2: $z = \dfrac{4a}{5}$